The idea of an item being rare due to emergent properties reminds me of something cool from Diablo 1. There was at least one unique item that shared a "kind" s

The idea of an item being rare due to emergent properties reminds me of something cool from Diablo 1. There was at least one unique item that shared a "kind" slot with another, lower level unique. Due to a bug, the game would always choose the lesser item. This might have made the other item not just rare but actually impossible to get, except for an interaction with another feature: if a unique dropped once in a game, the same unique would not drop again. This mechanism took the lower level unique out of the running, allowing the other one to drop - but only if this same kind of unique were chosen twice per game. So the probability of finding the second unique was P(A|B). That is, the probability of finding it in any random game was probability of A given B. Out of the four billion 32-bit seed integers the random number generator for Diablo 1 might be initialized with, only a handful satisfied this requirement.